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ip default-network


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#1 jcdenton12

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Posted 24 January 2011 - 03:57 AM

hi
I am reading CCNP ROUTE foundation library and this command is driving me crazy

so let us image a simple problem
I need to insert default route into EIGRP autonomous system but I am too lazy to configure it manually on every router

I can do it by propagating summary route of 0.0.0.0 0.0.0.0 into system and it works fine
I can do it by creating static route of 0.0.0.0 0.0.0.0 that points to the interface (it does not work when it points to next-hop IP) and add network statement for 0.0.0.0 0.0.0.0 into EIGRP router and it works fine

I should be able to do it using ip default-network command
according to book there are only two important things to be aware of
use classful network in this command
every router in EIGRP domain has to have that route that is marked as candidate default in its routing table
--> so you either put network statement for it or redistribute it into EIGRP autonomous system

but it does not work for me
since I am using not classful network but only a subnetwork of it (1.1.1.0/24)

when I use ip default-network 1.0.0.0 , that route is marked with * in the routing table but gateway of last resort is not set
if I use ip default-network 1.1.1.0 after that, gateway of last resort is set so I can pring anything from this router only

but it is not propagating into EIGRP system to other routers
book says it should work
while googling I found this postwith CCNA lab where it apparently works


I attach pkt file with all 3 methods (2 work fine, ip default-network does not) (it is in zip file because pkt extension is not allowed here)
I tried to lab it in GNS3 and there was the same unsuccessful result in the end

someone please tell me what is the mistake that I made because I am crazy thanks to this command

I look forward to your replies
Martin

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#2 MarkinManchester

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Posted 24 January 2011 - 04:10 AM

Hi

Are you looking for something like "ip summary-address eigrp xxx 0.0.0.0 0.0.0.0" where this in turn generates Null0 ??


Mark
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#3 jcdenton12

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Posted 24 January 2011 - 04:31 AM

Hi

Are you looking for something like "ip summary-address eigrp xxx 0.0.0.0 0.0.0.0" where this in turn generates Null0 ??


Mark


yes, when I use this command alone, it genereated default route pointing to Null0 and this route is propagated into EIGRP autonomous system
however, if I create static default route and then use this command, no Null0 is created (because ip route 0.0.0.0 0.0.0.0 points to valid outgoinng interface) and then it works like expected


what I am looking for is the explanation what I did wrong and why my ip default-network is not working as expected

thank you

PS: there are 3 sets of routers with different ways of propagating default route in that zip->pkt file
you can see what my ip default-network command looks like and hopefully tell me what is wrong with it
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#4 chrcel

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Posted 24 January 2011 - 04:44 AM

I see actually ip default-network works counter intuitively. you have to a route to the default-network in the routing table for it to work. It will than take the gateway to than network as default gateway.
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#5 jcdenton12

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Posted 24 January 2011 - 05:16 AM

so what is wrong with my config in PacketTracer / GNS3?

reading this did not help me either
please tell me what is wrong


I tried it with RIPv2 with the same topology and it worked as expected with ip default-network

Edited by jcdenton12, 24 January 2011 - 05:19 AM.

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#6 chrcel

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Posted 24 January 2011 - 05:27 AM

I don't have packet tracer to look at configs.
http://www.cisco.com...shtml#ipnetwork

It might be that EIGRP just does not default-network. I somehow find it easier to use 0.0.0.0/0 instead...
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#7 jcdenton12

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Posted 24 January 2011 - 05:45 AM

I don't have packet tracer to look at configs.
http://www.cisco.com...shtml#ipnetwork

It might be that EIGRP just does not default-network. I somehow find it easier to use 0.0.0.0/0 instead...


Yes I have read that
the problem is

if I configure that ip default-network on every router, that it works
but the idea behind this command is to configure it on the only one and then use routing protocol to propagate it to the other routers

so if I take screenshot of topology and send GNS3 configs, coul you look into that?
or anyone else?
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#8 chrcel

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Posted 24 January 2011 - 05:48 AM

I won't be able to test it any sooner than tomorrow about this time. But it got me interested :)
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#9 jcdenton12

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Posted 24 January 2011 - 06:05 AM

I won't be able to test it any sooner than tomorrow about this time. But it got me interested :)


OK
this one looks interesting
http://routemyworld....d-dynamips-lab/

just copy pasted config into GNS3 and voila
router B had gateway of last resort set


I am gonna investigate and dig into it more and hopefully I will find the magic behind the this
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#10 jcdenton12

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Posted 24 January 2011 - 06:49 AM

OK
so here is what I found out

when you use AUTO-SUMMARY then gateway of last resort will be set and it does propagate throughout the whole EIGRP domain

however if you use NO AUTO-SUMMARY, the route will be marked as candidate default, no gateway of last resort will be set on that router nor will the candidate default be propagated anywhere


when you use AUTO-SUMMARY, chances are you are not using classful IP address and therefore Null0 interface will be created for that
so you can reach network that you selected via ip default-network but nothing behind it, because it will be sent into Null0 interface


So I digged more into it

if you happen to have full class A or B or C assigned to your interfaces (what a waste) then no Null0 is created but no gateway of last resor is set either
on the other routers it stayed set, but since the next-hop did not have it set anything behind ip default-network was unreachable


So to sum it up

I don't think ip default-gateway provides all the capabilities that default route 0.0.0.0 0.0.0.0 does

you cannot reach anything behind NETWORK that you specify in ip default-network NETWORK
if you want to be able to reach NETWORK only, than you have to go classful which wastes to many IPs, IMHO

this is what foundation library says
(see attached images)

sure
it will work in topology shown
but it is not as full-featured as tradition default route is



could someone plase confirm me findings or post mistakes that I made (if any)
someone with experience of using ip default-network could share some insight of this command


Thank you very much in advance
Martin

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#11 laf_c

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Posted 24 January 2011 - 03:18 PM

OK
so here is what I found out

when you use AUTO-SUMMARY then gateway of last resort will be set and it does propagate throughout the whole EIGRP domain

however if you use NO AUTO-SUMMARY, the route will be marked as candidate default, no gateway of last resort will be set on that router nor will the candidate default be propagated anywhere


when you use AUTO-SUMMARY, chances are you are not using classful IP address and therefore Null0 interface will be created for that
so you can reach network that you selected via ip default-network but nothing behind it, because it will be sent into Null0 interface


So I digged more into it

if you happen to have full class A or B or C assigned to your interfaces (what a waste) then no Null0 is created but no gateway of last resor is set either
on the other routers it stayed set, but since the next-hop did not have it set anything behind ip default-network was unreachable


So to sum it up

I don't think ip default-gateway provides all the capabilities that default route 0.0.0.0 0.0.0.0 does

you cannot reach anything behind NETWORK that you specify in ip default-network NETWORK
if you want to be able to reach NETWORK only, than you have to go classful which wastes to many IPs, IMHO

this is what foundation library says
(see attached images)

sure
it will work in topology shown
but it is not as full-featured as tradition default route is



could someone plase confirm me findings or post mistakes that I made (if any)
someone with experience of using ip default-network could share some insight of this command


Thank you very much in advance
Martin


Nice work mate!

Unfortunately I am still very busy these days, but I will keep this in my mind and give it a try whenever possible.
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#12 peter.stando

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Posted 20 December 2013 - 05:39 AM

when I use ip default-network 1.0.0.0 , that route is marked with * in the routing table but gateway of last resort is not set
if I use ip default-network 1.1.1.0 after that, gateway of last resort is set so I can pring anything from this router only
 

 

 

I have same problem with IOS 15.1. Then I boot 12.4 and it works. 


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